The present invention relates to printing of printed circuit boards. More particularly, the present invention relates to offset printing of printed circuit boards.
Printed circuit boards have been manufactured by various techniques including chemical etching, polymer thick film deposition, screen printing and sputtering. Each of these methods have certain areas where they can be improved.
Drawbacks associated with chemical etching of printed circuit boards relate to the use of hazardous chemicals and long processing times during the manufacturing process. The disposal of spent chemicals is also of concern.
Polymer thick film is another method for manufacturing printed circuit boards including flexible membrane boards. While this method has been relatively successful, one drawback is that it does not produce as fine a line as chemical etching. Further, the use of a polymer thick film manufacturing method is generally limited to one ink at a time. It is also more labor intensive than various other methods.
The present invention overcomes the drawbacks of the foregoing manufacturing methods by providing a new manufacturing method which more efficiently produces improved printed circuit boards.
The present invention relates to a method of manufacturing printed circuit boards. A preferred method comprises providing a conductive composition and a substrate. The conductive composition may comprise a liquid vehicle and electrically conducive material suspended or dissolved in the liquid vehicle. The method also comprises the step of applying the conductive composition to the substrate through an offset lithography printing technique so that a desired printed circuit pattern is obtained on the substrate.
Preferably, the conductive composition is applied to the substrate under a pressure of at least about 4 psi. More preferably, the application pressure is between about 50 psi and 1000 psi. Even more preferably, the pressure may be between about 50-500 psi.
It is also preferable for the method of manufacturing printed circuit boards to comprise the step of applying conductive composition to a substrate through a cold welding process. As used herein, the concept of xe2x80x9ccold weldingxe2x80x9d the conductive composition to the substrate comprises applying the conductive composition to the substrate under a sufficient pressure so that the conductive material is separated from the liquid vehicle of the conductive composition. At the same time, the conductive material is forced together to form a conductive trace pattern on the surface of the substrate.
In certain embodiments, the substrate may comprise a substantially nonabsorbent material. Such nonabsorbent materials may include, but are not limited to, polyester, Kapton, or the like. When a substantially nonabsorbent substrate is used, a cold weld process takes place where the vehicle (i.e., the liquid portion of the conductive composition) gets squeezed out and the conductive particles (such as nonprecious or precious metals) form together. The liquid vehicle would be supported on the surface of the substantially nonabsorbent substrate and would then evaporate from such surface.
In an environment where the substrate is relatively absorbent, the application of pressure during the cold welding process will force the liquid vehicle to more quickly become absorbed into the substrate itself, while certain amounts of the liquid vehicle would still evaporate. Examples of a nonabsorbent substrate include, but are not limited to, a fibrous substrate that may be made of a paper product or the like, and porous coating such as porous ink, etc. The application of a suitable pressure in this embodiment will also force the conductive particles together to form a continuous conductive trace pattern with suitable conductivity.
In accordance with a preferred aspect of the present invention, the method of manufacturing printed circuit boards comprises initially applying conductive composition to a substrate through a printing process and subsequently subjecting said conductive composition and said substrate to a predetermined pressure after the conductive composition has been applied to the surface of the substrate. The predetermined pressure is preferably at least about 4 psi. More preferably, the predetermined pressure is between about 50 psi-1000 psi. Even more preferably, the predetermined pressure may be between about 50 psi-500 psi.
As used herein, the term xe2x80x9cprinted circuit boardxe2x80x9d is intended to cover various types of flexible and nonflexible substrates onto which a conductive trace pattern is applied. In accordance with the preferred method of the present invention, at least a portion of the conductive trace pattern comprises conductive composition printed on the surface of the substrate. It should also be understood that as used herein, the term xe2x80x9cprinted circuit boardxe2x80x9d is intended to cover substrates having a printed circuit pattern arranged on the surface thereof either with or without active or inactive circuit components also secured to the substrate. Thus, the term xe2x80x9cprinted circuit boardxe2x80x9d is intended to cover what may be considered only a portion of a printed circuit board as conventionally understood. The term xe2x80x9cprinted circuit boardxe2x80x9d is also intended to cover complete printed circuit boards that have circuit components thereon as conventionally understood. It should also be understood that as used herein, the term xe2x80x9cprinted circuit boardxe2x80x9d is intended to cover various types of substrates having conductive material arranged thereon, such as membrane switches and other types of switch devices.
The method of manufacturing printed circuit boards in accordance with the present invention is intended to cover several embodiments including, but not limited to, applying conductive composition to the surface of a substrate where pressure is applied as the conductive composition itself is applied to the surface of the substrate. The method is also intended to cover an in-line pressure application system where the conductive composition is initially printed on the surface of a substrate and pressure is later applied as part of an in-line manufacturing process. In another embodiment, the method is intended to cover printing conductive composition on the surface of a substrate and applying pressure to the conductive composition and the substrate off-line (i.e., in a separate assembly remote from the printed circuit board manufacturing assembly). Examples of the three foregoing embodiments are shown in the following drawings that form part of this disclosure.
Conductive compositions, such as conductive inks can be printed through various techniques such as offset lithography, letterpress, gravure, flexography, and the like to obtain very fine circuits for printed circuit boards. Simple single layer circuits as well as multilayered circuits can be achieved using the offset or letter press process. The substrates on which the conductive compositions are printed, can be fed into an associated printing press by one or more sheets at a time, or as part of a continuous web where a roller feed mechanism is employed.
Various printing techniques may be employed. For example, the conductive composition may be printed on a substrate through gravure, flexography, lithography, letter press, hot stamping, offset printing, and other methods. Offset printing is one preferred method as it often results in a completed circuit board with very fine conductive lines which have smooth edges. Further offset printing can be used to produce half-tones with controlled dot gain.
Various existing offset ink formulations and other coating compositions that are modified to be conductive, can be used. For example, ultraviolet (UV) cured offset inks and other types of chemistry, depending upon the substrate being printed and the desired properties, can be used. Most any ink or other printable composition, modified to be printed via an offset press, letter press, gravure, flexo and/or coaters"" can be used regardless of chemistry. The preferred embodiment is UV cured inks because they can be cured immediately after the conductive ink is applied, thus eliminating picking by the next blanket or offsetting onto the sheet stacked directly on top of the printed substrate. Existing sheet fed and web presses can be employed and the limitation to the width or length of the substrate is that of the machinery it is printed on. The substrates are carriers for the printed circuit board and can be of any known content.
Substrates can range from paper to various plastics and synthetics as well as laminates of various kinds. The substrates may have a pressure sensitive layer thus allowing it to be mounted onto other substrates such as fiberglass for traditional handling of the circuit including, but not limited to, drilling, soldering, die cutting and otherwise adhering or attaching discreet components to the circuit board.
Self substrates may be made by printing or coating various layers in the desired sequence, and printed onto a release liner, thermo degradable substrate, moisture degradable substrate or otherwise degradable thus making a self substrate. The printed layers may be peeled off from the substrate or otherwise detached from the substrate.
Conductive inks can be applied via an offset lithography, letterpress or a combination press or a fine line pressure sensitive or heat activated adhesive, can be printed, that can act as a contact adhesive so that when a conductive foil or dry film comes in contact with the printed surface by hot or cold laminating, it would pull off the conductive dry film thus giving a continuity to the conductive lines that are formed. Either way would be effective in making conductive circuits that can withstand a pre-engineered set of specification for resistance levels, adhesion characteristics, ultimate properties, size and shape, etc.
Line widths and spacing of the printed conductive compositions may vary from greater than that of traditional thick film printing to at least as fine as conductive lines obtained through a chemical etched process. Lines may also be printed many layers high, depending upon practicality and registration constraints. The improvement of high speed printing in register is a major advancement for multi-layer printing. For example, circuits 20 or more layers thick would be acceptable.
Some objects and advantages of the present method of manufacturing printed circuit boards is that it reduces the manufacturing cost from that of conventional printed circuit board production, reduces or eliminates environmental pollutants from the chemical etching circuit board process, prints smoother lines, prints less ink when desired, prints cleaner edges, and reduces production time. Other objects of this invention is to be able to print short and long runs, gang different circuit boards together in one run and change substrates that are being printed upon without incurring additional expense for set ups. The present method would also reduce ink usage and allow for high speed curing in-line. Splitting the ink fountain by having different inks being printed at the same time allows for the printing of different inks on the same plate being deposited on the same substrate at the same or different time. This can help in scheduling and reduce costs for set ups and plate costs.
Offset or letterpress printing would also allow for in line die cutting, depending on the machinery capabilities, so that the circuit is complete when finished as opposed to off line cutting of the circuits which increases handling and costs. Either method can be used in high speed manufacturing.
Another object of the invention is to reduce the cost of membranes as well and to give new capabilities for size and sensitivity. These membranes could be used as sensors for level sensing, potentiometers, motion sensors, temperature sensors, humidity sensors, pressure sensors, light sensors, barometric sensors, touch sensors, magnetic sensors, bend sensors, smoke sensors, sound sensors, circuit breakers, and various other uses.
Another object of the invention is to revolutionize and cut the cost of battery testers, produce inexpensive capacitors and resistors, reduce costs for magnets, antennas both sending and receiving, EMI shielding, heating elements, electrolumniscent lamps, batteries, and other rigid and flexible circuitry printed on Offset, letterpress, gravure, flexo and other printing and coating methods.